1. Field of the Invention
This invention relates to optical waveguides, and, more particularly, to optical waveguides, modulators and switches utilizing the photo-elastic effect in combination with the electro-optic effect.
2. Description of the Prior Art
Optical waveguides have been described which utilize an optically transparent crystal such as gallium arsenide (GaAs), lithium tantalate (LiTaO.sub.3) or lithium niobate (LiNbO.sub.3) and which form a channel for guiding light wave energy by applying a bias voltage to electrodes on the material, thereby effecting a change in the refractive index of the material. The refractive index change leads to the formation of an optical waveguide in a region wherein the refractive index is increased relative to the surrounding area. Similarly, this phenomenon has also been observed in a region wherein the material is biased so as to cause a decrease in the relative refractive index, when two such biased regions bound a normal region. This effect is called the electro-optic effect and is due to the change in the dielectric constant and hence the refractive index of the crystal in response to the applied bias voltage. Light travelling in a medium having a transverse variation in refractive index is reflected towards regions having the larger refractive index.
The localized region may be defined by a pair of spaced electrodes disposed on a planar surface of electro-optical material in side-by-side relationship, as disclosed by D. J. Channin in U.S. Pat. No. 3,795,433 for Voltage Induced Optical Waveguide Means, issued March, 1974, or with electrodes disposed on opposing surfaces of the electro-optic material, as disclosed by M. Furukawa in U.S. Pat. No. 3,965,745 for Light Wave Guide Circuit, issued Oct. 3, 1972.
Electro-optic responsive crystals, such as lithium niobate (LiNbO.sub.3) and lithium tantalate (LiTaO.sub.3), can be characterized by positive or negative electro-optic coefficients, so that, by selecting an appropriate bias potential, with choice of the plane of the crystal on which the electrodes are deposited and suitable poling of the material, the refractive index in the waveguide region may be caused to increase or decrease as desired. Thus, by proper choice of the electrode disposition and corresponding voltage excitation, electro-optic materials have been utilized to provide modulators and switches as well as waveguides. See for example, U.S. Pat. No. 4,145,109, Electro-Optic Multiplexing With High Interchannel Isolation, issued Mar. 20, 1979, invented by the present inventor and assigned to the assignee of the present invention.
A disadvantage of the prior art electro-optic devices is their relatively low sensitivity, requiring applied potentials as high as 400 volts between electrodes. Because of the extremely thin wafers of materials used and the close proximity of the electrodes, arcing between electrodes frequently results, resulting in degradation or destruction of the device. While both direct current and alternating current may be applied to the electrodes, high dc voltage gradients may permanently change the refractive index of the crystal, while ac voltages are effective for only one half of the wave cycle. Further, LiNbO.sub.3 is susceptible to damage to the crystalline structure at high optical excitation levels.
Other waveguides have been formed by diffusing a transition metal such as titanium (Ti) into a LiNbO.sub.3 crystal substrate to form a guiding layer of increased refractive index. Permanent waveguides are thereby formed without the need for applied bias voltage. Such waveguides can be formed by evaporating a thin layer of the metal on the surface of the crystal and then heating the crystal to a suitable temperature for diffusion.
In preparing a crystal substrate for electro-optic use, it is first "poled" by the well-known technique of heating the material above the Curie temperature and cooling in an electric field. This aligns the molecular structure so as to polarize the crystal, thereby rendering it sensitive and responsive to the imposition of an applied electric field or diffused metal stripes and hence exhibiting the electro-optic effect. If heated again beyond the Curie temperature, the material will be depoled, thus losing its desirable electro-optic properties. It has been found that a material such as LiTaO.sub.3, which is preferable for use as a substrate due to its lesser susceptibility to optical damage, is depoled by the diffusion process, since the diffusion requires a temperature of 1100.degree. C. while the Curie temperature of LiTaO.sub.3 is 600.degree. C. Diffusion into LiTaO.sub.3 at temperatures below 600.degree. C. is feasible but very slow. Further, Ti-diffused guides have substantial losses of approximately 1 dB per cm. Such losses limit the optical performance of the waveguide and degrade the performance of devices such as modulators and switches. It is a characteristic of integrated optic modulators and switches that they can be made to operate at lower voltages and hence provide increased sensitivity if the device is made longer to increase the coupling area. However, if the waveguide is inherently lossy, the increased attenuation due to the increased device length outweights the prospect of improved performance. Modulators and switches have been constructed in recent years using such Ti-diffused waveguides and LiNbO.sub.3 substrates, with the coupling of light between adjacent waveguide channels controlled by a voltage bias applied to selected electrodes. However, the applied voltage required for 100% modulation or complete switching has still proved to be excessive for many applications. Further, the diffusion of metal ions such as Ti into LiNbO.sub.3 has been effectively limited to utilization in single-mode guides that are of the order of a wavelength in width. For multi-mode structures, which are preferable from some viewpoints, such as the availability of coupling devices, the required cross-sectional area of the guide is much larger and only the voltage induced refractive index change phenomena has been used.
Recently, photo-elastic guides were reported using evaporated metal or SiO.sub.2 stripes on LiTaO.sub.3 and LiNbO.sub.3 substrates. These guides were observed to offer real advantages over the previously used voltage-induced guides and single-mode Ti-diffused guides on LiNbO.sub.3, as reported by the present inventor in Photo-Elastic Waveguides in LiTaO.sub.3, and LiNbO.sub.3, Appl. Opt. 19, 3423 (1980). In devices using the photo-elastic effect, the waveguide is caused, at least in part, by the effect of changing the refractive index by the stress field in the semi-conductor material surrounding a deposited stripe, or in a window formed between a plurality of such stripes. This stress field results from the state of compression or tension induced by the deposited film due to the differing thermal expansion coefficients of the substrate and the deposited film and the elevated temperature required for deposition. Using the photo-elastic effect, it is possible with the correct pattern of an evaporated film to produce regions of increased refractive index that will guide light. The previously disclosed work by the present inventor used evaporated films of gold with a chromium flash to promote adhesion. These films had moderately large stress values initially; however, with time the stress faded due to relaxation of the film and, accordingly, the photo-elastic properties deteriorated.